An OpenGL example with support for Linux/Unix/BSD and Emscripten.

opengl_emscripten_example.cpp

/*
    Build instructions:

    To compile and run the application with G++ or Clang++ use:
    $ CXX -o application.o -c opengl_emscripten_example.cpp
    $ CXX -o application application.o -lGLEW -lGLU -lGL -lglfw
    $ ./application
    
    To compile and with Emscripten use:
    $ em++ -o application.o -c opengl_emscripten_example.cpp -s MAX_WEBGL_VERSION=2
    $ em++ -o application.html application.o -lGLEW -lGL -lglfw -s USE_GLFW=3
    $ emrun --no_browser application.html
*/

#include <GL/glew.h>
#include <GLFW/glfw3.h>

#include <iostream>

#ifdef __EMSCRIPTEN__
#include <emscripten.h>

GLFWwindow *glfwMainWindow;
GLuint mainShaderProgram;
GLuint globVertArrObj;
GLuint scalarID;
float scalar = 0.5f;
float scalarFactor = 0.00390625f;
#endif

const char *vertexShaderSource =
#ifndef __EMSCRIPTEN__
    "#version 330\n"
    "layout (location = 0) in vec3 aPos;\n"
    "uniform float scalar;\n"
    "void main()\n"
    "{\n"
    "    gl_Position = vec4(scalar * aPos, 1.0f);\n"
    "}\0";
#else
    "attribute vec3 aPos;\n"
    "uniform float scalar;\n"
    "void main()\n"
    "{\n"
    "    gl_Position = vec4(scalar * aPos, 1.0);\n"
    "}\0";
#endif

const char *fragmentShaderSource =
#ifndef __EMSCRIPTEN__
    "#version 330\n"
    "out vec4 FragColor;\n"
    "void main()\n"
    "{\n"
    "    FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
    "}\0";
#else
    "void main()\n"
    "{\n"
    "    gl_FragColor = vec4(1.0, 0.5, 0.2, 1.0);\n"
    "}\0";
#endif

void framebuffer_size_callback(GLFWwindow *window, int width, int height);
void processInput(GLFWwindow *window);

// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;

#ifdef __EMSCRIPTEN__
void glfwIter()
{
    // scale the triangle
    if (scalar == 1.0f)
    {
        scalarFactor = -0.00390625f;
    }
    if (scalar == 0.5f)
    {
        scalarFactor = 0.00390625f;
    }
    scalar += scalarFactor;
    glUniform1f(scalarID, scalar);
    // input
    // -----
    processInput(glfwMainWindow);

    // render
    // ------
    glClear(GL_COLOR_BUFFER_BIT);
    glUseProgram(mainShaderProgram);
    glBindVertexArray(globVertArrObj);
    glDrawArrays(GL_TRIANGLES, 0, 3);

    // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
    // -------------------------------------------------------------------------------
    glfwSwapBuffers(glfwMainWindow);
    glfwPollEvents();
}
#endif

int main()
{
    // glfw: initialize and configure
    // ------------------------------
    if (!glfwInit())
    {
        std::cerr << "Failed to initialize GLFW" << std::endl;
        return -1;
    }
    // glfwWindowHint(GLFW_SAMPLES, 4);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    // glfw window creation
    // --------------------
    GLFWwindow *window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "OpenGL Example", NULL, NULL);
    if (window == NULL)
    {
        std::cout << "Failed to create GLFW window" << std::endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
    glClearColor(0.2f, 0.3f, 0.3f, 1.0f);

    // initialise GLEW
    glewExperimental = true;
    if (glewInit() != GLEW_OK)
    {
        std::cerr << "Failed to initialize GLEW" << std::endl;
        glfwTerminate();
        return -1;
    }

    // vertex array object
    GLuint vertArrObj;
    glGenVertexArrays(1, &vertArrObj);
    glBindVertexArray(vertArrObj);

    // triangle vertices
    float triangleVertices[] = {
        -0.5f, -0.5f, 0.0f, //
        0.5f, -0.5f, 0.0f,  //
        0.0f, 0.5f, 0.0f    //
    };
    GLuint triangleVertBuf;
    glGenBuffers(1, &triangleVertBuf);
    glBindBuffer(GL_ARRAY_BUFFER, triangleVertBuf);
    glBufferData(GL_ARRAY_BUFFER, sizeof(triangleVertices), triangleVertices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void *)0);
    glEnableVertexAttribArray(0);

    // triangle shaders
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    int success;
    char info_log[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, info_log);
        std::cout << "Error while compiling vertex shader: " << info_log << std::endl;
        return -1;
    }
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, info_log);
        std::cout << "Error while compiling fragment shader: " << info_log << std::endl;
        return -1;
    }
    GLuint shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
    if (!success)
    {
        glGetProgramInfoLog(shaderProgram, 512, NULL, info_log);
        std::cout << "Error while linking shader: " << info_log << std::endl;
        return -1;
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

    // render loop
    // -----------
#ifndef __EMSCRIPTEN__
    float scalar = 0.5f;
    float scalarFactor;
    GLuint scalarID = glGetUniformLocation(shaderProgram, "scalar");
    while (!glfwWindowShouldClose(window))
    {
        // scale the triangle
        if (scalar == 1.0f)
        {
            scalarFactor = -0.00390625f;
        }
        if (scalar == 0.5f)
        {
            scalarFactor = 0.00390625f;
        }
        scalar += scalarFactor;
        glUniform1f(scalarID, scalar);

        // input
        // -----
        processInput(window);

        // render
        // ------
        glClear(GL_COLOR_BUFFER_BIT);
        glUseProgram(shaderProgram);
        glBindVertexArray(vertArrObj);
        glDrawArrays(GL_TRIANGLES, 0, 3);

        // glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
        // -------------------------------------------------------------------------------
        glfwSwapBuffers(window);
        glfwPollEvents();
    }
#else
    glfwMainWindow = window;
    mainShaderProgram = shaderProgram;
    globVertArrObj = vertArrObj;
    scalarID = glGetUniformLocation(shaderProgram, "scalar");
    emscripten_set_main_loop(glfwIter, 60, 1);
#endif

    // glfw: terminate, clearing all previously allocated GLFW resources.
    // ------------------------------------------------------------------
    glfwTerminate();
    return 0;
}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
// ---------------------------------------------------------------------------------------------------------
void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        glfwSetWindowShouldClose(window, true);
}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes
// ---------------------------------------------------------------------------------------------
void framebuffer_size_callback(GLFWwindow *window, int width, int height)
{
    // make sure the viewport matches the new window dimensions; note that width and
    // height will be significantly larger than specified on retina displays.
    glViewport(0, 0, width, height);
}